From the copending applications mentioned above and the references set out herein and various publications which may be mentioned below, it should be apparent that is is known to provide electronic switching units of the contactless or proximity type, i.e. which are responsive to the approach of an actuating influence, generally some body or influence which may affect the frequency or output of a variable frequency oscillator which can constitute the sensing element of the proximity or contactless switch.
The sensor may work into a switching amplifier, i.e. an electronic element responsive to the proximity of the body or influence and which, in turn, controls an output element, generally an output transistor, which can close a load circuit containing the load, e.g. an alarm relay, and a source of electric current in series therewith.
The current supply for the electronic circuit can include a MOS-FET power transistor serving as a constant-voltage generator or as a constant-current generator, the MOS-FET power transistor and the output transistor being connected in series with one another.
Contactless electronic switches of this type are finding increasing use in applications where only the electrical, mechanical and electromechanical switching devices have been used heretofore and indeed wherever actuating contact may be problematical, and mechanical contacts which must physically engage one another may be undesirable or disadvantageous for wear, environmental-deviation and like reasons. Measuring, control and regulation circuits utilize the contactless or proximity-switching devices in increasing numbers.
In general such proximity switches serve to indicate the approach of the affecting element to which the proximity switch may be responsive and, indeed, the approach of the affecting element to a predetermined critical point. Such approach is detected and serves to switch the state of the output transistor so that, for example, a normally blocking output transistor will be rendered conductive or a normally conductive output transistor can be switched into a blocking state depending upon whether closed or open circuit operation of a load is desired.
While I have specifically mentioned proximity of an affecting element to the sensor, thereby indicating that such switches are primarily responsive to proximity to an object to the sensor, this expression should be understood to also include a change in a physical parameter which has an analogous effect on the sensor so that the electronic proximity switching device can also be utilized as a contactless switch for control or other purposes responsive to the physical parameter.
The sensor itself can be an inductively or capacitively influenced oscillator when the proximity switch is to form an inductive or capacitive proximity switch. The sensor may also be constituted by or can include a photoresistor, a photodiode or a phototransistor when the proximity switch is of the optoelectronic type.
Inductive proximity switches, for example, may use an oscillator which oscillates at a fixed frequency as long as a metal article has not approached to within a predetermined distance. The oscillation is affected with an amplitude of the oscillator voltage which generally will lie above a threshold value. When however, the metal article reaches the predetermined distance from the oscillator, the oscillator is subjected to an increased damping and either the oscillation terminates, or the frequency changes, or the amplitude of the oscillator voltage falls below the threshold value.
With capacitive proximity switches, the oscillator generally does not oscillate, or oscillates with a value below a threshold value of the amplitude of the oscillator voltage as long as an affecting element does not increase the capacity between a sensing electrode and a counter electrode sufficiently. When the affecting element, however, approaches within a predetermined distance, the greater capacity thus resulting between the sensing electrode and the counter electrode causes oscillation of the oscillator and an increasing amplitude, above a threshold value of the oscillator voltage. In both cases, the state of the output transistor is dependent upon the amplitude of the oscillator voltage, either directly or via a switching amplifier which is so responsive.
Optoelectronic proximity switches have a light source and a light receiver and may also define a light curtain, i.e. a path crossed by the light beam such that when an article interrupts this path, the contactless switch responds.
One can distinguish between two light-curtain systems. In one such system, the light source and the light receiver, e.g. a photoreceiver, phototransistor or photodiode, are located on opposite sides of the path to be monitored. In another type of light-curtain system, the source and the sensor are located on the same side of the path and, on the opposite side of the path, a reflector may be provided to reflect the light from the emitter to the detector. In both cases, the monitoring of the path involves detecting an interruption in the light curtain by an article traveling along the path.
In yet a third type of photosensitive or optoelectronic contactless switch, the receiver only receives light which is reflected from the affecting element.
As previously mentioned, many, if not all, of these switching systems can have an external line connected with one pole of the operating voltage source and only a single other external line which is connected to the load, the load being connected in series with the source, i.e. to the other pole thereof.
With such systems, the current supply and voltage supply for the sensor and the switching amplifier is not free from problems because the feed voltage or feed current must be applied not only in the conducting state of the switching device but also in the blocking state thereof.
In the present description, I will frequently refer to the supply current or the supply voltage and, indeed, these terms refer to interchangeable concepts. When the supply current falls, so does the supply voltage and it is optional whether current or voltage control is used in most circuits.
When the current supply is drawn at less in part from the operating current source, the switching between the conductive state of the output transistor to the blocking state or vice versa, may result in a substantial change in the feed current or feed voltage available for the sensing components and even the switching amplifier of the circuit.
Indeed, the function of the proximity switch can best be insured by guaranteeing that in the conductive state, there is no significant voltage drop resulting from the conductive state and that in the blocking state there is practically no residual current flowing in the load circuit.
It should be appreciated that with earlier systems utilizing only two external lines as mentioned, it was practically impossible to avoid either a severe voltage drop in the conductive state or a significant residual current in the blocking state. As a consequence, these disadvantages had to be accepted as compromises in such systems.
The problem of reducing the voltage drop in the conductive state of the switch has been dealt with in German patent documents Nos. 19 514 137, 21 27 956, 26 13 423 and 27 11 877.
Electronic switching devices of this kind permit a reduction in the voltage drop in the conductive state, and in other situations it has been proposed to supply the feed current or voltage from a feeder circuit which is constituted by a DC/DC converter because with such a DC/DC converter, the secondary circuit can have a higher converter voltage than the primary circuit so that the voltage drop in the conductive state of the switch can be low, below the requisite feed voltage for the proximity sensor and, if desired, the switching amplifier (see German patent documents-open applications Nos. 28 08 156, 29 22 309 and 33 20 975).
Electronic switching units of the type with which the present invention is concerned can be direct-current switching units, alternating-current switching units or AC/DC switching units, i.e. switching units in which the operating voltage source can be either alternating current or direct current.
With direct-current switching units, it is customary to provide an output transistor as the controlled electronic switch operated by the proximity sensor. With alternating-current switching units, however, the output element controlled by the sensor may an output thyristor.
Since an electronic-switching element having an output thyristor can only be used in conjunction with a direct-current operating voltage source when it is of the gate-turnoff type, for AC/DC switching devices, the electronic output-switching element used is generally an output transistor.
When the switching device has an output transistor, generally one must use a power transistor to be able to carry the requisite load current. Power transistors, however, have problems when they are included in circuits with high operating voltages, of, for example, 220 volts AC which is generally rectified to pulsating DC.
It is known to provide in series with such an output transistor, as will be detailed more fully below, a MOS-FET power transistor. In this case, the MOS-FET power transistor in the blocking state of the swith, can carry the high operating voltage which would otherwise be applied exclusively across the output transistor, the MOS-FET being peculiarly effective because of its construction to serve in this manner as a high-voltage element.
In the state of the art over which the invention is an advance, the voltage drop when the unit switches to the conductive state is relatively high and can be, for example, about 10 volts. As previously noted, such significant voltage drops in switching from one state to the other can be detrimental to the operation and can affect adversely proximity detection, and consequently the response of the circuit.